Insight to Potential of TiB₂ and CeO₂ Inoculants on Microstructural Evolution in Laser Powder Bed Fusion Processed Superalloy IN718

This investigation focuses on understanding the microstructural changes resulting from the presence of CeO₂ and TiB₂ inoculants in superalloy Inconel 718 (IN718) processed by laser powder bed fusion (L-PBF). It was demonstrated that additions of 1 vol pct of CeO₂ or TiB₂ inoculant blended with the powder feedstock effectively refined the grain structure of IN718. Interestingly, compared to the TiB₂ inoculant particles that were largely unchanged in the melt pool, the CeO₂ particles reacted with the melt, forming Ce(Al, Ti)O₃ oxides that melted and agglomerated along the melt pool boundaries and surface during the L-PBF process. The presence of CeO₂ and Ce(Al, Ti)O₃ oxide particles in the melt pool contributed to decreasing the cooling rate during solidification. As a result, the as-built grain structure was noticeably finer in L-PBF IN718 containing TiB₂ compared to those containing CeO₂, processed under the same parameters. Detailed characterization of the as-built microstructures revealed that the TiB₂ inoculant particles did modestly react with the IN718 and formed minor amounts of nano-borides formed in the vicinity of TiB₂ particles. Following a solution heat treatment of 2 hours at 1150 °C, the overall fraction of borides in the microstructure increased, effectively pinning grain boundaries and preventing grain growth. These borides contributed to retaining the as-built grain structure and significantly increased the microhardness by ~ 85 HV_0.5 compared to L-PBF processed IN718 without inoculant addition.